1. Field of the Invention
The invention relates to the removal of contaminants from streams of ammonia gas and liquid. More particularly it relates to the production of substantially contaminant-free streams of ammonia gas or liquid for use in the production of semiconductors and similar products which cannot tolerate the presence of such contaminants during manufacture.
2. Background Art
There is current research to develop high performance light emitting diodes (LEDs). Such LEDs are intended for such disparate uses as in outdoor displays, vehicles, traffic signals, lasers, medical devices and indoor lighting, and are expected to replace the current bulbs or fluorescent lighting tubes. The high performance LEDs emit light of specific wavelengths compared to broad spectrum wavelength emissions of prior bulbs and tubes, so that optimal light spectra can be provided for each end use by combination of LEDs of different colors. For instance, LED combinations with spectra matching incandescent lighting (as compared to the spectra of flourescent lighting) can be provided for indoor use, without creating the heat generation and dispersion problems caused by incandescent bulbs.
These LEDs are made by metal organic chemical vapor deposition (MOCVD) using materials such as gallium-, aluminum gallium- and indium gallium nitrides and phosphides. In a process currently of significant interest, gallium nitride is deposited from a gaseous mixture of ammonia, hydrogen and trimethyl gallium. Similar, gallium nitride is being considered for “blue lasers,” i.e. lasers which emit blue light. Because blue light has a shorter wavelength than red, yellow or green light, blue lasers are anticipated to be capable of forming compact disks which will have a much higher information density than is presently the case with compact disks produced with red laser light. Gallium nitride for such blue lasers would be manufactured in the same type of ammonia/hydrogen/trimethyl gallium gaseous environment as described above for the high capacity LEDs.
The LEDs, blue lasers and integrated circuits are all manufactured with electron accepting p-type dopants. Such products are extremely sensitive to the presence of electron-donating n-type materials, and very small concentrations of such n-type are sufficient to deactivate the p-type dopants and impair or destroy the performance and operability of the integrated circuits, LEDs and blue lasers. Oxygen is a particularly efficient n-type material, and the presence of molecular oxygen causes lattice defects and is detrimental to the desired band gap properties in the semiconductor or laser material. Even very low concentrations of oxygen (<100 ppb, usually <50 ppb, even <10 ppb) can be sufficient to cause sufficient reduction in performance or operability (especially in wavelength control) so as to require discarding of the product after manufacture or to significantly shorten operating lifetime. Similar detrimental effects are observed with similar low concentrations of water or hydrocarbons in the manufacturing system.
There are also numerous manufacturing processes of current interest in which ammonia is a major component in the production of high purity products. Commonly these processes use ammonia in gaseous form, but liquid ammonia is also used to some extent. In addition, a liquid ammonia component frequently is vaporized during a process to be used subsequently in gaseous form. A common requirement in these processes is that all reactants, catalysts, carriers, etc. must have the least practical contaminant level, since the products produced must be of very high purity. Examples include the following.
Recent advances in integrated circuit semiconductor technology have included the development of semiconductors with copper interconnects instead of aluminum interconnects. Copper interconnects are advantageous in that copper has less resistance than aluminum, which leads to higher performance in microprocessors, microcontrollers and random access memories. However, copper tends to migrate over a period of time, so it is necessary to construct barrier layers in the semiconductor to prevent the copper migration. Such boundary layers are typically made of nitrides such as tantalum nitride, titanium nitride or silicon nitride. These layers are commonly formed by deposition from a hydride gas, e.g., ammonia.
Ammonia is widely used as a source of nitrogen for film development in some thin film applications. The ammonia allows for lower temperature film growth in chemical vapor deposition (CVD) processes.
As mentioned, in addition to oxygen contamination, the presence of water vapor, gaseous hydrocarbons and/or carbon dioxide gas in hydride gases such as ammonia is also detrimental, since those materials lead to degradation of the products formed by deposition of active layers of metals or metal compounds from a hydride gas environment. Water is one of the most common and yet most difficult impurities to remove from the gases. Water is of course ubiquitous in almost all ambient environments. Even systems which are nominally referred to as “dry” usually have significant amounts of water, and most drying processes can reduce the moisture content of a gas only to a “minimum” which is still in the parts per million (ppm) range. However, since for many purposes water contents in the ppm range are quite acceptable, there are numerous patents and articles in the literature dealing with such types of “ppm drying processes.”
In the manufacture of such products, moisture contents of the depositing gases which are in the ppm range are excessively wet. To form satisfactory products, the water content of the depositing gases must be reduced to the parts per billion (ppb) range, usually down to no more than about 100 ppb. See Whitlock et al, “High Purity Gases,” in Ruthven, ed., ENCYCLOPEDIA OF SEPARATION TECHNOLOGY, vol. 1, pp. 987-1000 (1997).
Attempts to use materials such as reduced nickel or copper catalysts to remove contaminants such as oxygen, carbon dioxide and water from hydride gases have not been successful. While contaminant removal can be effected for short periods of time down to the 10 ppb level, the reactive effects of the hydride gases, especially ammonia, very quickly cause the materials to degrade and contaminate the gas stream with metal complexes. Though pre-existing impurities may be reduced, the introduction of new impurities to the manufacturing process is unacceptable.
Processes have been described in which oxygen has been removed from ammonia streams by metals serving as “getters.” However, these have been relatively ineffective at reaching sufficiently low levels of decontamination. In addition, the getters are deposited on substrates, such as silica or zeolites, which do not play a central role in the decontamination process, and also may themselves be degraded by the hydride gases. See, for instance, U.S. Pat. Nos. 5,496,778 (Hoffman et al.), 5,716,588 (Vergani et al.) and 4,976,944 (Pacaud et al.); PCT publication No. WO 97/06104 (SAES Getters S.p.A.); and European Patent No. EP 0 784 595 B1 (SAES Getters S.p.A.). In particular, some of these references teach that manganese:iron ratios of >2:1 as depositions on such substrates are detrimental to getter performance and are to be avoided. The references specifically teach that very low manganese:iron ratios, usually about 0.012-0.16:1, are to be preferred. Further, the reference processes are usually not effective for removal of carbon dioxide or water, as compared to oxygen, from ammonia gas streams.
Consequently, the problem of removal of contaminant levels down to ≦100 ppb from ammonia remains a significant problem in the field of production of high purity LEDs, blue lasers, semiconductors, and the like. Those processes which are being used are expensive because of the very short service life of the decontaminating materials and the need for their frequent replacement. In addition, since it is difficult to determine the exact rate of deterioration of the decontaminating materials in the presence of the ammonia, users of such decontaminating materials must schedule their discard and replacement at intervals less than the shortest expected service life. To do otherwise would risk failure of a decontamination unit with the resultant loss of contaminated product when the excessive contaminant concentrations reaches the production chamber through the failed unit. Consequently, the current systems require that many if not most of the decontamination units must be discarded while they still have some degree of useful service life left, thus further increasing the expense of the system operations. This must be considered against the background that the market expects that manufacturing processes must have a continuous pure gas flow at high flow rate with consistent levels of purification and low cost of ownership. Therefore the more that a process deviates from these anticipated parameters, the less acceptable it is. The successful processes are those which have sufficiently high levels of purification to permit the maximum practical operating lives at reasonable cost.
While ammonia decontamination systems have been described that include multiple adsorbent beds or vessels that alternate in decontaminating and regenerative function, such systems suffer drawbacks in that they either a) require the periodic replacement of adsorbent, e.g., a getter alloy, and/or b) involve a regenerative process that relies on the administration of gas from outside the system to generate fresh adsorbent, e.g., alumina, from contaminant-saturated adsorbent. See, e.g., U.S. Pat. No. 5,833,738 (Carrea et al.).
Until the present invention, the above processes were limited by the need to add exogenous supplies of gas, for instance from cylinders, to the system. This required more handling and plumbing, which in turn exacted a toll on overall process efficiency.
If the supply of regenerant gas or fresh adsorbent could be supplied from within the system itself, it would simplify and enhance existing purification and gas delivery systems, thereby resulting in lower operating costs, which savings in turn could be passed on to the benefit of the consumer.